In any given size reduction application, the specific properties of the material being processed play a key role in how finished particle size reduction is achieved. Hardness, brittleness, moisture content, oil content, etc. are all considered when determining not only the appropriate style of size reduction equipment, but also the configuration of the equipment's internal components.

70% Control

In the majority of hammer mill applications, the key factor determining finished particle size is the the screen. Any material that enters the grinding chamber must be reduced to a size small enough to pass through the screen that covers the mill's discharge opening. Because of this the screen size provides 70% of the control over the finished particle size.

Size reduction takes place when material is fed into a hammer mill's grinding chamber and it is repeatedly struck by flailing ganged hammers that are attached to a rotor spinning at very high speed. A combination of hammer blows, collision with the walls of the grinding chamber, and particle on particle impact reduce the material until it is able to pass through the screen.

Sizing up Screens

Screens and bar grates are constructed from steel and are available with perforations (screens) or spaces (bar grates) in a broad range of sizes. Screen size is determined by the size of the openings in the screen, and is described in the following units of measure: inches, millimeters, microns (one millionth of a meter), and US mesh (the number of wires running east/west and north/south in one square inch of screen).

The appropriate screen size is determined by the desired finished particle size, and the properties of the material being processed. That is, characteristics such as friability and moisture content have an effect of the manner in which a material will break down. As a result, using the same screen to process materials of different properties will result in a range of different finished particle sizes.

For example:

This variation is called particle size distribution, and it is based on the the individual properties of the materials being processed.

In this example: Glass is very friable, and will shatter very easily upon impact. In comparison, green wood chips are a fibrous material with a moisture content of up to 50%, which both effect the ease with which they are reduced. Finally, computer hard drives are very hard and comprised mostly of metals, making them comparatively hard to process and unlikely to breakdown beyond the screen or bar grate size.

The Force Factor

But screen size only accounts for 70% of what determines the finished particle size. The reamining 30% is attributed to the force of the impact on the material being processing. In the case of hammer mills, force is determined by rotor speed, and the size and number of hammers.

Let's take a closer look, this time using the example of a drinking glass:

Rotor Speed: Slowly tap the glass with a hammer and it will break into perhaps 3 to 4 large pieces. Conversely, if you hit it with the same hammer at a rapid speed, it will break into many more, much smaller pieces.

Hammer Size: Strike a water glass with a butter knife, and it will break into a few large pieces. Strike the same glass with a sledge hammer, and it will shatter into 1000+ pieces.

In short:

Summary

Finished particle size is determined by a combination of screen size, rotor speed, and the size and number of hammers. Material must remain in the grinding chamber until it is able to pass through the screen covering the hammer mill's discharge opening. Optimal screen size is determined by the desired finished particle size, and the properties of the material being processed.

Material is fed in through the top of the mill. Once in the grinding chamber, it is reduced by a combination of repeated hammer blows, particle on particle impact, and contact with the walls of the mill. The material will remain in the grinding chamber until it is reduced to a size that is able to pass through the screen covering the mill's discharge opening.

The simplicity of this design makes it a very versatile hammer mill, one that can be adapted to suit a wide variety of materials, such as:

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Similar, Yet Different

The same hammer mill for fishmeal and coal? Well, yes and no. The basic framework of the mill is the same. However, the configuration of the variable components is how they differ. That determination is based on the following criteria:

Hammer size and style - Number of hammers, size, style and metallurgy.

Screens or bar grates - Style and thickness of screen or bar grates, and size of openings.

Choice of proper RPM

It's Optional

Finally, once the the mill is configured, the last determination is whether or not any optional peripheral equipment is needed. For this, the following questions must be answered:

How will the material be fed into the mill? By hand, auger, or belt conveyor?

How will the material be taken from the mill? Heavy materials such as stone or metal may evacuate via gravity, while light or low density materials will require pneumatic suction.

Is dust a concern?

Answers to these questions will help to determine the best types of optional equipment such as belt conveyors, augers, rotary feeders, and dust collection, as well as the most efficient design of the infeed and discharge chutes.

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If you were looking for the latest magic bullet for trimming your waistline (and aren't we all?) unfortunately, Google has lead you astray. Material size reduction is our topic of discussion today. There are many terms that fall under this umbrella: crush, grind, pulverize, shred, de-lump, de-fiberize, just to name a few. Likewise, there is a whole host of machinery styles that accomplish these goals: hammer mills, grinders, shredders, lumpbreakers, impactors, jaw crushers and more.

Hammer mills operate on the basic principle that most materials will crush, shatter or pulverize upon impact. This is accomplished by a simple four step process:

Material is fed into the mill, typically by gravity.

Inside the grinding chamber, the material is repeatedly struck by flailing ganged hammers which are attached to a shaft that rotates at a specified speed. The material is crushed by a combination of hammer blows, collision with the walls of the grinding chamber and particle on particle impacts.

Perforated metal screens or bar grates cover the discharge opening of the mill retain the coarse material for further processing while allowing properly sized material to pass through.

Hard, heavy material such as stone, glass or metal can exit the mill via gravity. Lighter or low density materials such as wood and paper require pneumatic suction for effective discharge.

One size does not fit all

Well, that would be too easy, wouldn't it? Finished particle size is determined by a combination of the following: screen (or bar grate) size, shaft speed and hammer configuration.

For example, a fast shaft speed, small screen and large number of hammers typically produces a fine end product. Conversely, a larger screen, fewer hammers and slower shaft speed will result in a coarse product. Disclaimer alert: It is important to know that this is a very simplistic explanation of a very complex engineering process. Each of these factors is determined based on careful consideration of the the material being processed and the user's production goals.

Each of the key components: screen size, shaft speed and hammer configuration can be changed individually or in combination to achieve the precise finished particle size at the desired production rate.